1. Field of the Invention
The invention relates to an internal combustion engine and a manufacturing method therefor and, more particularly, to an internal combustion engine in which an anodic oxide film is formed on part or all of a wall surface facing a combustion chamber of the internal combustion engine and a manufacturing method for an internal combustion engine, which has a characteristic in a method of forming the anodic oxide film.
2. Description of Related Art
An internal combustion engine, such as a gasoline engine and a diesel engine, is mainly formed of an engine block, a cylinder head and a piston. A combustion chamber of the internal combustion engine is defined by a bore face of the cylinder block, a top face of the piston assembled in the bore, a bottom face of the cylinder head and top faces of intake and exhaust valves arranged in the cylinder head. With high-power requirements to recent internal combustion engines, it is important to reduce the cooling losses of the internal combustion engines. As one of measures to reduce the cooling losses, there is a method of forming a heat insulation film made of ceramics on an inner wall of the combustion chamber.
However, because the above-described ceramics generally have a low thermal conductivity and a high thermal capacity, there occurs a decrease in intake efficiency or knocking (abnormal combustion due to remaining of heat in the combustion chamber) due to a steady increase in surface temperature. Therefore, the ceramics have not presently become widespread as a film material for the inner wall of the combustion chamber.
For this reason, the heat insulation film that is formed on the wall surface of the combustion chamber is desirably formed of a material having not only heat resistance and heat insulation properties as a matter of course but also a low thermal conductivity and a low thermal capacity. That is, in order not to increase the wall temperature steadily, the film should have a low thermal capacity in order to reduce the wall temperature following a fresh air temperature in an intake stroke. Furthermore, in addition to the low thermal conductivity and low thermal capacity, the film is desirably formed of a material that can resist against explosion pressure at the time of combustion in the combustion chamber, injection pressure, and repeated stress of thermal expansion and thermal shrinkage and that has a high adhesion to a base material, such as the cylinder block.
Focusing on an existing known technique, Japanese Patent Application Publication No. 58-192949 (JP 58-192949 A) describes a piston, in which an alumite layer is formed on a top face and a ceramic layer is formed on the surface of the alumite layer, and a manufacturing method for the piston. With this piston, the alumite layer is formed on the top face, so the piston has an excellent heat resistance property and an excellent heat insulation property.
In this way, with the alumite layer (anodic oxide film) formed on a wall surface facing a combustion chamber of an internal combustion engine, it is possible to form the internal combustion engine having an excellent heat insulation property, a low thermal conductivity and a low thermal capacity. In addition to these capabilities, an excellent swing characteristic is also an important capability that is required for the anodic oxide film. The “swing characteristic” is a characteristic that the temperature of the anodic oxide film follows the gas temperature in the combustion chamber although the anodic oxide film has a heat insulation capability.
Incidentally, when the above-described anodic oxide film is observed microscopically, the anodic oxide film has such a structure that a large number of cells are adjacent to each other, a large number of cracks are present on the surface of the anodic oxide film, part of the cracks extend inward (that is, in the thickness direction of the anodic oxide film or substantially the thickness direction), and a large number of internal defects extending in a direction different from the thickness direction (a horizontal direction perpendicular to the thickness direction or substantially the horizontal direction) are present in the film. The inventors, et al. identified that these cracks and internal defects are micropores having a micro-size diameter (or a maximum diameter in cross section) of about the range of 1 μm to 100 μm. The “cracks” are originated from crystallized products of casting aluminum alloy.
There are also a large number of small pores (nanopores) having a nano-size diameter inside the anodic oxide film in addition to the above-described micro-size cranks and internal defects. Generally, the nanopores are also present so as to extend from the surface of the anodic oxide film in the thickness direction of the anodic oxide film or substantially the thickness direction. The “nanopores” are originated from anodizing and are regularly arranged.
In this way, the anodic oxide film to be formed generally has micropores, such as surface cracks and internal defects having a micro-size diameter or maximum size in cross section and a large number of nano-size nanopores.
The inventors, et al. describe a technique that relates to an internal combustion engine in which an anodic oxide film having a low thermal conductivity, a low thermal capacity, an excellent heat insulation property and an excellent switch characteristic is provided on part or all of a wall surface facing a combustion chamber and a manufacturing method for the internal combustion engine in Japanese Patent Application Publication No. 2013-060620 (JP 2013-060620 A). More specifically, a large number of nanopores are formed in a state where a sealant does not penetrate into the nanopores by applying porous sealing treatment to nano-size small pores present inside the anodic oxide film formed on the wall surface facing the combustion chamber, thus keeping at least part of the nanopores from being sealed. Subsequently, a sealant is applied to relatively large micro-size gaps, thus sealing at least part of the gaps with a seal converted from the above sealant. Thus, an internal combustion engine in which the anodic oxide film having an excellent heat insulation property, a high strength and an excellent swing characteristic is provided on part or all of the wall surface facing the combustion chamber.
With the internal combustion engine and the manufacturing method therefor, described in JP 2013-060620 A, a predetermined porosity is ensured because the nanopores are not sealed, and this guarantees the heat insulation property. However, it is difficult to ensure a sufficient porosity, because pores that are not seated are nanopores. Therefore, it is required to increase the thickness of the anodic oxide film in order to guarantee the heat insulation property. For example, it is possible to form an anodic oxide film having an excellent heat insulation property by setting the thickness of the anodic oxide film to about 300 to 500 μm; however, forming an anodic oxide film having such a thickness takes a manufacturing time, causing an increase in manufacturing cost.